1. Field of the Invention
The present invention relates to a method and apparatus for measuring chain and for synchronizing the operation of conveyor apparatus, such as a discharge mechanism, with movement of the conveyor carriers by tracking the carriers through their drive chain.
2. Description of the Prior Art
In a typical conveyor system, such as the one shown in the U.S. Pat. No. 3,448,870 of J. R. Gallo et al, a plurality of spaced apart carriers are driven around an endless path by an endless chain. The carriers are loaded (frequently while moving) at one or more loading stations, and discharged (also while moving) at selected discharge stations. A common method of directing a particular carrier to a desired discharge station includes an escort code member mounted on the carrier and set by an operator with a code unique to the desired discharge station. A code reader at the discharge station activates a discharge mechanism to remove the load on the fly from the carrier in response to proximity with, or engagement by, the escort code member.
As conveyor systems become larger, the expense of providing escort code mechanism for each carrier, and providing a code reader for each discharge station, becomes excessive. Moreover, with an increasing amount of code apparatus, the likelihood of malfunctions increases significantly.
Another method of actuating mechanism to discharge a load from a moving carrier at a selected discharge station utilizes a signal carrier which is driven in synchronism with the carriers. The signal carrier may, for example, be a magnetic tape, as shown in the U.S. Pat. No. 2,825,476 of D. C. Muller, which has signals therein to produce a discharge signal in a selected pick-up head. One problem with a system of this type is maintaining a sufficiently close synchronism between the movement of the carriers and the movement of the signal carrier.
Counting means have been used to track the carriers in a conveyor system. In one previously described system, a sensor, which scans the teeth of a sprocket, generates pulses which advance through a circuit in synchronous timed relation with the carriers (see U.S. Pat. No. 3,815,723). In another system relating to a letter sorting machine, it has been known to use a shaft encoder attached to the drive chain sprocket of the letter cart conveyor to generate a pulse to correspond to the linear advance of the conveyor (see U.S. Pat. No. 3,696,946). In any system where the teeth of a sprocket or the revolutions of a shaft are counted, slack due to wearing or stretching of a conveyor chain can substantially reduce the reliability of the system. It has been found that the length of the conveyor chain of a fixed number of links varies significantly because of manufacturing tolerances, change in chain tension which causes stretch, and wear between pins and bushings.
It will be noted that prior art systems which count or sense sprocket teeth or sprocket shaft rotation to track a carrier, rely on the assumption that links or segments of the chain extending between the point of measurement and the carrier remain constant in length. In effect, the assumed position of the carrier is based on the standard, or nominal chain link length.
It should also be noted that in the prior art, means have been provided to compensate for inaccuracies in the tracking of the carrier. In U.S. Pat. No. 3,815,723, a proximity switch is provided to sense carrier trays adjacent the unloader so that the latter will be actuated in the precise timed relation to the tray to be unloaded. In doing so, any slack or other inaccuracy in the chain that might place the tray in the wrong position relative to the unloader will be overridden and corrected.
Briggs et al, U.S. Pat. No. 3,180,995, shows the measurement of a moving object.